The present invention relates to a thermal head which is suitably used for a thermal printer.
A typical example of a portion of the thermal head according to the prior art is illustrated in FIGS. 1 and 2, in which an aluminum oxide substrate 1 has a glass glaze layer 2 coated over its upper face. Coated over the upper face of the glass glaze layer 2 is a resistance heating element 3, over which are formed electrodes 4a, 4b at a predetermined distance, thus a portion of the heating element 3, which is located below the gap between the electrodes 4a, 4b serving as a heating portion 3a. One of the electrodes 4a, 4b is grounded and the other is connected to an output terminal of a power supply control unit (not shown), which supply electric current to the heating portion 3a. The reference numeral 5 indicates an oxidation-resistant film to cover both the electrodes 4a, 4b and the heating portion 3a. The oxidation-resistant film 5 is coated with a wear- or abrasion resistant film 6. The oxidation-resistant film 5 and the abrasion-resistant film 6 constitute a protection film. FIG. 2 illustrates a plan view of the heating portion 3a when the protection layer is removed. When current is supplied to each of heating portions 3a of the thermal head, it is uniformly heated, so that the shape of a dot of a picture is reproduced as shown by the broken line in FIG. 2, for example, in a thermal paper.
In the thermal printer, it is preferable to change the size of dots, reproduced on a printing paper, for expressing a picture in gradation. However, the thermal head above described cannot make such dot printing, and the printing thereof is restricted by the shape of each heating portion 3a.
Accordingly, it is an object of the present invention to provide a thermal head which is capable of changing the size of dots of a picture in printing.